In the commercial production of plastics, elastomers, man-made fibers, adhesives, and surface coatings, a tremendous variety of polymers are used. There are many ways to classify these compounds. For example, polymers can be categorized according to whether they are formed through chain-growth or step-growth reactions. Alternatively, polymers can be divided between those that are soluble in selective solvents and can be reversibly softened by heat, known as thermoplastics, and those that form three-dimensional networks that are not soluble and cannot be softened by heat without decomposition, known as thermosets. Additionally, polymers can be classified as either made from modified natural compounds or made from entirely synthetic compounds.
A logical way to classify the major commercially employed polymers is to divide them by the composition of their monomers, the chains of linked repeating units that make up the macromolecules. Classified according to composition, industrial polymers are either carbon-chain polymers (also called vinyls) or heterochain polymers (also called noncarbon-chain, or nonvinyls). In carbon-chain polymers, as the name implies, the monomers are composed of linkages between carbon atoms; in heterochain polymers a number of other elements are linked together in the monomers, including oxygen, nitrogen, sulfur, and silicon.
By far the most important industrial polymers are polymerized olefins, which comprise virtually all commodity plastics. Olefins, also called alkenes, are unsaturated hydrocarbons (compounds containing hydrogen [H] and carbon [C]) whose molecules contain one or more pairs of carbon atoms linked together by a double bond. The olefins are classified in either or both of the following ways: (1) as cyclic or acyclic (aliphatic) olefins, in which the double bond is located between carbon atoms forming part of a cyclic (closed-ring) or an open-chain grouping, respectively, and (2) as monoolefins, diolefins, triolefins, etc., in which the number of double bonds per molecule is, respectively, one two, three, or some other number.
Generally, olefin molecules are commonly represented by the chemical formula CH2═CHR, where C is a carbon atom, H is a hydrogen atom, and R is an atom or pendant molecular group of varying composition. The composition and structure of R determines which of the huge array of possible properties will be demonstrated by the polymer.
More specifically, acyclic monoolefins have the general formula CnH2n, where n is an integer. Acyclic monoolefins are rare in nature but are formed in large quantities during the cracking of petroleum oils to gasoline. The lower monoolefins, i.e., ethylene, propylene, and butylene, have become the basis for the extensive petrochemicals industry. Most uses of these compounds involve reactions of the double bonds with other chemical agents. Acyclic diolefins, also known as acyclic dialkenes, or acyclic dienes, with the general formula CnH2n-2, contain two double bonds; they undergo reactions similar to the monoolefins. The best-known dienes are butadiene and isoprene, used in the manufacture of synthetic rubber.
Olefins containing two to four carbon atoms per molecule are gaseous at ordinary temperatures and pressure; those containing five or more carbon atoms are usually liquid at ordinary temperatures. Additionally, olefins are only slightly soluble in water. Olefins have traditionally been produced from alkanes by fluid catalytic cracking (FCC) or steam cracking, depending on the size of the alkanes. Heavy olefins are herein defined as containing at least five carbon atoms and are produced by FCC. Light olefins are defined herein as containing one to four carbon atoms and are produced by steam cracking. Alkanes are similar to alkenes, except that they are saturated hydrocarbons whose molecules contain carbon atoms linked together by single bonds. The simplest alkanes are methane (CH4, the most abundant hydrocarbon), ethane (CH3CH3), and propane (CH3CH2CH3). These three compounds exist in only one structure each. Higher members of the series, beginning with butane (CH3CH2CH2CH3), may be constructed in two or more different ways, depending on whether the carbon chain is straight or branched. Such compounds are called isomers; these are compounds with the same molecular formula but different arrangements of their atoms. As a result, they often have different chemical properties.
In the conversion of alkanes to alkenes, fluid catalytic cracking and steam cracking (direct catalytic dehydrogenation processes) are known to have their drawbacks. For example, the processes are endothermic, meaning that heat is absorbed by the reactions and the temperature of the reaction mixtures decline as the reactions proceed. This is known to lower the product yield, resulting in lower value products. In addition, coke forms on the surface of the catalyst during the cracking processes, covering active sites and deactivating the catalyst. During regeneration, the coke is burned off the catalyst to restore its activity and to provide heat needed to drive the cracking.
This cycle is very stressful for the catalyst; temperatures are high and fluctuate and coke is repeatedly deposited and burned off. Furthermore, the catalyst particles are moving at high speed through steel reactors and pipes, where wall contacts and interparticle contacts are impossible to avoid.
While it may be easy to dismiss catalyst damage and loss in less expensive catalysts, the catalysts used in FCC and steam cracking units are quite expensive. The expense stems from the use of precious metals. For example, a typical supported metal catalyst may cost in the range of $20-$40 per pound, of which the cost of the precious metals may be between 50-80%. Thus, for a reactor that uses 2 million pounds of catalyst, the total cost of the metals in the reactor is considerable. Further, because FCC and steam cracking units are large and require steam input, the overall processes are expensive even before taking catalyst cost into consideration.
As a result, because olefins comprise the most important building blocks in modern petrochemical industry, the development of alternate routes other than FCC and steam reforming have been explored. One such route is oxidative dehydrogenation (ODH). In ODH, an organic compound is dehydrogenated in the presence of oxygen. Oxygen may be fed to the reaction zone as pure oxygen, air, oxygen-enriched air, oxygen mixed with a diluent, and so forth. Oxygen in the desired amount may be added in the feed to the dehydrogenation zone and oxygen may also be added in increments to the dehydrogenation zone. However, catalysts for oxidative dehydrogenation are still being investigated and the development of more effective catalysts for ODH is highly desirable.